Communications system including control means for designating communication between space nodes and surface nodes

ABSTRACT

A cellular communications system is provided have both satellite nodes and surface nodes for providing mobile cellular communications services for a plurality of mobile user units. The surface and satellite nodes are fully integrated by a network controller for providing service over large areas. In the alternative, each user unit includes link control means permitting the user unit to designate the mode of communications, either satellite node communications or ground node communications. In addition, multiple beam, relatively high gain antennas are disposed in the satellite nodes to establish satellite cells having enough gain in the satellite part of the system such that a user unit need only comprise a small, mobile handset with a non-directional antenna for communications with both ground nodes and satellite nodes.

RELATED APPLICATIONS

This application is a continuation application of copending U.S.application Ser. No. 08/751,651, filed Nov. 18, 1996, which is, in turn,a continuation application of U.S. application Ser. No. 08/255,341,filed Jun. 7, 1994, now abandoned, which is, in-turn, acontinuation-in-part of U.S. application Ser. No. 08/145,246, filed Oct.28, 1993, now U.S. Pat. No. 5,446,756, which is, in-turn, acontinuation-in-part of U.S. application Ser. No. 07/781,972, filed Oct.24, 1991, now U.S. Pat. No. 5,339,330, derived from PCT Application S/NPCT/US91/01852, filed Mar. 19, 1991, which was, in-turn, acontinuation-in-part of U.S. application Ser. No. 07/495,497, filed Mar.19, 1990, now U.S. Pat. No. 5,073,900.

BACKGROUND

This invention relates to improvements in mobile wireless communicationsystems. In particular, the invention relates to communication systemssuch as a cellular mobile communications system having integratedsatellite and ground nodes.

In another respect the invention pertains to mobile wirelesscommunications systems which can locate and/or disable thecommunications equipment of a fraudulent user of the system.

According to another aspect the invention concerns methods and apparatusfor minimizing interference due to passive intermodulation (PIM) ofradiated energy in a satellite using a single large antenna forcommunication to and from a user's transceiver.

In yet another respect the invention pertains to a multi-node wirelesscommunications systems provided with methods and protocols for seamlesshand-over of a user from one node to another.

The cellular communications industry has grown at a fast pace in theUnited States and even faster in some other countries. It has become animportant service of substantial utility and because of the growth rate,saturation of the existing service is of concern. High density regionshaving high use rates, such as Los Angles, New York and Chicago are ofmost immediate concern. Contributing to this concern is the congestionof the electromagnetic frequency spectrum which is becoming increasinglysevere as the communication needs of society expand. This congestion iscaused not only by cellular communications systems but also by othercommunications systems. However, in the cellular communications industryalone, it is estimated that the number of mobile subscribers willincrease on a world-wide level by an order of magnitude within the nextten years. The radio frequency spectrum is limited and in view of thisincreasing demand for its use, means to more efficiently use it arecontinually being explored.

Mobile communications system such as Specialized Mobile Radio (SMR), theplanned Personal Communications Service (PCS) and existing cellularradio are primarily aimed at providing mobile telephone service toautomotive users in developed metropolitan areas. For remote area users,airborne users, and marine users, AIRFONE and INMARSAT services existbut coverage is incomplete and/or service is relatively expensive.Mobile radio satellite systems in an advanced planning stage willprobably provide improved direct-broadcast voice channels to mobilesubscribers in remote areas but still at significantly higher cost incomparison to existing ground cellular service. The ground cellular andplanned satellite technologies complement one another in geographicalcoverage in that the ground cellular communications service providesvoice and data telephone service in relatively developed urban andsuburban areas but not in sparsely populated areas, while the plannedearth orbiting satellites will serve the sparsely populated areas.Although the two technologies use the same general area of the RFspectrum, they are basically separate and incompatible by design as theypresently exist. At present, if a user needs both forms of mobilecommunications coverage, he must invest in two relatively expensivesubscriber units, one for each system.

The demand for mobile telephone service is steadily expanding and withthe expansion of the service, the problem of serving an increased numberof subscribers who are traveling from one region to another has becomeof primary importance. Cellular communications systems divide theservice areas into geographical cells, each served by a base station ornode typically located at its center. The central node transmitssufficient power to cover its cell area with adequate field strength. Ifa mobile user moves to a new cell, the radio link is switched to the newnode provided there is an available channel. However, if the mobile usertravels into a region where all channels are busy, or that is not servedby cellular service, or, in some cases, into an area served by adifferent licensee/provider, then his call may be abruptly terminated.

Present land mobile communication systems typically use a frequencymodulation (FM) approach and because of the limited interferencerejection capabilities of FM modulation, each radio channel may be usedonly once over a wide geographical area encompassing many cells. Thismeans that each cell can use only a small fraction of the totalallocated radio frequency band, resulting in an inefficient use of theavailable spectrum. In some cases, the quality of speech is poor becauseof the phenomena affecting FM transmission known as fading and "deadspots". The subjective effect of fading is repeated submersion of thevoice signal in background noise frequently many times per second if themobile unit is in motion. The problem is exacerbated by interferencefrom co-channel users in distant cells and resultant crosstalk due tothe limited interference rejection capability of FM. Additionally,communications privacy is relatively poor; the FM signal may be heard byothers who are receiving that frequency.

In the case where one band of frequencies is preferable over others andthat one band alone is to be used for mobile communications, efficientcommunications systems are necessary to assure that the number of usersdesiring to use the band can be accommodated. For example, there ispresently widespread agreement on the choice of L-band as thetechnically preferred frequency band for the satellite-to-mobile link inmobile communications systems. In the case where this single band ischosen to contain all mobile communications users, improvements inspectral utilization in the area of interference protection and in theability to function without imposing intolerable interference on otherservices will be of paramount importance in the considerations ofoptimal use of the scarce spectrum.

The spread spectrum communications technique is a technology that hasfound widespread use in military applications which must meetrequirements for security, minimized likelihood of signal detection, andminimum susceptibility to external interference or jamming. In a spreadspectrum system, the data modulated carrier signal is further modulatedby a relatively wide-band, pseudo-random "spreading" signal so that thetransmitted bandwidth is much greater than the bandwidth or rate of theinformation to be transmitted. Commonly the "spreading" signal isgenerated by a pseudo-random deterministic digital logic algorithm whichis duplicated at the receiver.

By further modulating the received signal by the same spreadingwaveform, the received signal is remapped into the original informationbandwidth to reproduce the desired signal. Because a receiver isresponsive only to a signal that was spread using the same uniquespreading code, a uniquely addressable channel is possible. Also, thepower spectral density is low and without the unique spreading code, thesignal is very difficult to detect, much less decode, so privacy isenhanced and interference with the signals of other services is reduced.The spread spectrum signal has strong immunity to multipath fading,interference from other users of the same system, and interference fromother systems.

In a satellite communications system, downlink power is an importantconsideration. Satellite power is severely limited; therefore, thenumber of users of the satellite that can be accommodated, andconsequently the economic viability of such a system, is in inverseproportion to how much satellite transmitter power must be allocated toeach user. Many of the proposed mobile communications satellite systemshave relied upon user antenna directivity to provide additionaleffective power gain. This has resulted in significant user equipmentexpense and the operational inconvenience of having to perform somesteering or selection of the antenna to point at the satellite.Additionally, hand held transceivers are impractical because of therelatively large directive antennas required.

In some ground cellular service, the user transceiver commonly radiatesat a power level which is 30 to 40 dB greater than is required on theaverage in order to overcome fading nulls. This results in greatlyincreased intersystem interference and reduced battery life. It wouldalso be desirable to provide a power control system to compensate forfading and interference without exceeding the minimum amount of powernecessary to overcome such interference.

Additionally, a user position determination capability would be usefulfor certain applications of a cellular communications system such astracking the progress of commercial vehicles en route. A further use maybe to provide users with an indication of their own position. Such acapability would be more useful with increased accuracy.

Thus it would be desirable to provide a cellular communications systemwhich integrates satellite nodes with surface nodes to provide coverageof greater surface areas without requiring the use of two differentsystems with attendant expense and hardware requirements. Additionally,it would be desirable to provide a cellular communications system usinga spread spectrum technique which can make more efficient use ofexisting frequency spectrum resources and result in increased privacy incommunications. Additionally, it would be desirable to permit the use ofa relatively low power, compact and mobile user handset having a small,non-directional antenna, one which can communicate with both theland-based stations and the satellite-based stations.

Further, it would be desirable to reduce or eliminate fraudulent use ofa wireless system by improved detection and location of the fraudulentuser.

In addition, it would be desirable to minimize interference due toPassive Inter Modulation of radiated energy in a satellite using asingle large antenna for communication to and from the user's handset.

Furthermore, it would be desirable to develop a protocol for seamlesshand over of a user from one system node to another.

SUMMARY OF THE INVENTION

The invention provides improvements in wireless communications systems.While various aspects of the invention will be explained by reference,for example, to a cellular communications system using spread spectrumwaveforms, it will be apparent to those skilled in the art that thesetechniques are applicable to similar forms of wireless communicationssystems, such as, for example, Specialized Mobile Radio (SMR), theplanned Personal Communications Service (PCS) and existing cellularradio systems.

The invention provides a cellular communications system having bothsurface and space nodes which are fully integrated. Areas where surfacenodes are impractical are covered by space nodes.

Space nodes comprise satellites which establish cells which in manycases overlap ground cells. Relatively high gain, multiple beam antennasare used on the satellites to produce sufficient gain in the system suchthat the user unit comprises only small, mobile handset with a smallnon-directional antenna.

A system network control center is used to coordinate system-wideoperations, to keep track of user locations, to perform optimumallocation of system resources to each call, dispatch facility commandcodes, and monitor and supervise over all system health. This systemnetwork control center is itself of a hierarchal nature comprising asystem network control center, regional node control centers whichcoordinate the detailed allocation of ground network resources within aregion, and one or more satellite node control centers responsible forallocation of resources among the satellite network resources.

In a preferred embodiment, the invention provides improvements in suchwireless communications systems, for example, a cellular communicationssystem using spread spectrum waveforms. The spread spectrum system makespossible the use of very low rate, highly redundant coding without lossof capacity to accommodate a large number of users within the allocatedbandwidth.

Briefly, in an additional aspect, the invention is directed to awireless communications system which includes node means and a pluralityof user units, each said user unit including a means for establishingselective communication between the node and the user unit. Such asystem is improved by establishing the geographical location of theselected user with the known locations of authorized users and denyingservice to the selected user if the selected users location does notcorrespond to one of the known locations of authorized users.Preferably, the system includes means for determining the position of aselected user unit by providing a timing signal to the selected userunit from the node, providing a timing response signal from the selecteduser unit from the node, providing a time response signal from theselected user unit in response to each timing signal, receiving thetiming response signal by the node, measuring the response time of theuser unit to the timing signal based on receipt of the timing responsesignal, and determining the position of the user unit based on the roundtrip time of transmission of the timing signal and receipt of the timingresponse signal.

In a more detailed aspect of the invention, the position means comprisesmeans for measuring the response times of the user unit to respectivetiming signals transmitted by at least two nodes and for determining theposition of the selected user unit based on the round trip times fromeach timing signal transmitting surface node.

In yet another aspect, the position means comprises means fordetermining the position of the selected user unit by measuring at aplurality of nodes the response time of the user unit to a timing signaltransmitted by at least one of the nodes and determining the position ofthe selected user unit based on the times of receipt by the nodes of thetiming response signal from the user unit.

In still another aspect, the position means may store a prioriinformation about the selected user unit and may determine the positionof the selected user unit by providing a timing signal to the user unitfrom a node, measuring the response time of the user unit to the timingsignal at the node, and determining the position of the user unit basedon such measurement and on the a priori information. Additionally, theposition means also determines in which cell a selected user unit is andindicates the location of the cell.

In yet another aspect of the invention, an adaptive transmitter powercontrol system and method compensate for received signal strengthvariations, such as those caused by building, foliage and otherobstructions. A path loss measure is derived from the received signalstrength and from data included in each transmitted signal whichindicates that transmitter's output power level. Based on the derivedpath loss and the transmitter's power level data, the receiver can thenadjust the power output of its own associated transmitter accordingly.

In yet a further aspect, each receiver determines the quality of thereceived signal and provides a local quality signal to its associatedtransmitter in the respective transceiver indicative of that receivedsignal quality. Each transmitter also transmits the local quality signalprovided to it from its associated receiver and the transceiver isadditionally responsive to the quality signal received from the othertransceiver with which it is in communication to control its own outputpower in the response to that quality signal.

In a more detailed aspect, the error rate of the received signal isdetermined in providing the quality signal, and in another aspect, thesignal-to-noise ratio (SNR) is measured to determine quality. Thetransceiver receiving the error rate signal or the SNR from the othertransceiver controls its own transmitter power output in response.

According to still another aspect of the invention, seamless hand-overof a mobile user from one system node to another is provided. Briefly,according to this aspect of the invention, in the operation of awireless communications system, which system includes a plurality ofnodes, a plurality of user units and means for establishing selectivecommunication between said first one of said nodes to a second one ofsaid nodes, the improvement comprising establishing an algorithm fordetermining a preferred node for said selective communication at anyselected time, periodically re-computing said algorithm, establishingcommunication between said first node and said second node when saidalgorithm computation indicates that said second node is the preferrednode, establishing call-initiation handshaking between said user andsaid second node, while maintaining said selective communication withsaid first node, establishing communications lock between said user andsaid second node, and interrupting communication between said user andsaid first node when said lock is established.

According to still another aspect of the invention, in the operation ofa satellite communications system, which includes at least two satellitenodes for separately receiving and transmitting signals to and from userunits, the improvement is provided for eliminating passiveintermodulation interference of the transmitted and received signals,comprising transmitting all node-transmitted signals from the antenna ofa first one of said satellite nodes and receiving all of saidnode-received signals by a second one of said satellite nodes. Inanother embodiment, the passive intermodulation interference iseliminated by time-duplexing the signals transmitted to and receivedfrom each satellite's antenna. In yet another embodiment, passiveintermodulation interference is reduced by assigning unique transmit andreceive subbands to each of the satellites.

Other aspects and advantages of the invention will become apparent fromthe following detailed description and the accompanying drawings,illustrating by way of example the features of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a)-(c) are diagrams showing an overview of the principalelements of typical communications systems which embody the principlesof the invention;

FIG. 2 is a diagram of the frequency sub-bands of the frequency bandallocation for a mobile system, e.g., a cellular system;

FIG. 3 is an overview block diagram of a communications system inaccordance with the principles of the invention without a networkcontrol center;

FIG. 4 is a diagram showing the interrelationship of the cellularhierarchial structure of the ground and satellite nodes in a typicalsection and presents a cluster comprising more than one satellite cell;

FIG. 5 is a block diagram of a satellite link system showing the userunit and satellite node control center;

FIG. 6 is a block diagram of one embodiment of a satellite signalprocessing in the system in FIG. 5;

FIG. 7 is a functional block diagram of a user transceiver showing anadaptive power control system;

FIGS. 8(a) through 8(h) show timing diagrams of an adaptive, two-waypower control system; and

FIG. 9 is a functional diagram of a two-way power control systemincorporating telemetered signal-quality deficiency supervisory control;

FIG. 10 explains the term Code Delay;

FIG. 11 depicts transmit and receive time slots when the satellite istime duplexed and the user is frequency duplexed;

FIG. 12 depicts transmit and receive time slots when the satellite andthe user unit are both time duplexed;

FIG. 13 is a block diagram showing an overview of the principal elementsof a communications system in accordance with the principles of theinvention wherein one satellite is used to transmit and a secondsatellite is used to receive the signals from the mobile user;

FIG. 14 is a diagram of the frequency sub-bands of the frequency bandallocation as modified in one embodiment to minimize the PIM with amultiple satellite system;

FIG. 15 is a generalized block diagram of a user unit with externalposition measurement;

FIG. 16 depicts a method of control hierarchy for a hybrid satellite andground based mobile communication system; and

FIG. 17 is a diagram of a protocol for hand-over of a mobile user fromone node to another.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As is shown in the exemplary drawings, the invention is embodied in amobile system, e.g., a cellular communications system utilizingintegrated satellite and ground nodes both of which use the samemodulation, coding, and spreading structure and both responding to anidentical user unit.

Referring now to FIG. 1(a), an overview of a communications system 10 ispresented showing the functional inter-relationships of the majorelements. The system network control center 12 directs the top levelallocation of calls to satellite and ground regional resourcesthroughout the system. It also is used to coordinate system-wideoperations, to keep track of user locations, to perform optimumallocation of system resources to each call, dispatch facility commandcodes, and monitor and supervise overall system health. The regionalnode control centers 14, one of which is shown, are connected to thesystem network control center 12 and direct the allocation of calls toground nodes within a major metropolitan region. The regional nodecontrol center 14 provides access to and from fixed land communicationlines, such as commercial telephone systems known as the public switchedtelephone network (PSTN). The ground nodes 16 under direction of therespective regional node control center 14 receive calls over the fixedland line network, encode them, spread them according to the uniquespreading code assigned to each designated user, combine them into acomposite signal, modulate that composite signal onto the transmissioncarrier, and broadcast them over the cellular region covered.

Satellite node control centers 18 are also connected to the systemnetwork control center 12 via status and control land lines andsimilarly handle calls designated for satellite links such as from PSTN,encode them, spread them according to the unique spreading codesassigned to the designated users, and multiplex them with othersimilarly directed calls into an uplink trunk, which is beamed up to thedesignated satellite 20. Satellite nodes 20 receive the uplink trunks,frequency demultiplex the calls intended for different satellite cells,frequency translate and direct each to its appropriate cell transmitterand cell beam, and broadcast the composite of all such similarlydirected calls down to the intended satellite cellular area. As usedherein, "backhaul" means the link between a satellite 20 and a satellitenode control center 18. In one embodiment, it is a K-band frequencywhile the link between the satellite 20 and the user unit 22 uses anL-band or an S-band frequency.

As used herein, a "node" is a communication site or a communicationrelay site capable of direct one or two-way radio communication withusers. Nodes may include moving or stationary surface sites or airborneor satellite sites.

User units 22 respond to signals of either satellite or ground nodeorigin, receive the outbound composite signal, separate out the signalintended for that user by despreading using the user's assigned uniquespreading code, de-modulate, and decode the information and deliver thecall to the user. Such user units 22 may be mobile or may be fixed inposition. Gateways 24 provide direct trunks that is, groups of channels,between satellite and the ground public switched telephone system orprivate trunk users. For example, a gateway may comprise a dedicatedsatellite terminal for use by a large company or other entity. In theembodiment of FIG. 1, the gateway 24 is also connected to that systemnetwork controller 12.

All of the above-discussed centers, nodes, units and gateways are fullduplex transmit/receive performing the corresponding inbound (user tosystem) link functions as well in the inverse manner to the outbound(system to user) link functions just described.

FIGS. 1(b) and 1(c) represent systems with space only and ground onlynodes. Certain aspects of this invention relate to these two systems aswell as the "hybrid" system previously described.

Referring now to FIG. 2, the allocated frequency band 26 of acommunications system is shown. The allocated frequency band 26 isdivided into 2 main sub-bands, an outgoing sub-band 25 and an incomingsub-band 27. Additionally the main sub-bands are themselves divided intofurther sub-bands which are designated as follows:

OG: Outbound Ground 28 (ground node to user)

OS: Outbound Satellite 30 (satellite node to user)

OC: Outbound Calling and Command 32 (node to user)

IG: Inbound Ground 34 (user to ground node)

IS: Inbound Satellite 36 (user to satellite node)

IC: Inbound Calling and Tracking 38 (user to node)

All users in all cells use the entire designated sub-band for thedescribed function. Unlike existing ground or satellite mobile systems,there is no necessity for frequency division by cells; all cells may usethese same basic six sub-bands. This arrangement results in a higherfrequency reuse factor as is discussed in more detail below.

In one aspect of the invention, the satellite 20 both receives signals(IS) and transmits (OS) signals to the user through a single largeantenna 62. If the transmit and receive frequencies are sufficiently farapart, Passive Inter Modulation (PIM) of the transmit signals will notcause perceptible distortion of the received signals. If, however, thetransmit and receive frequencies which are assigned are too closetogether, PIM of the transmit signals may cause distortion and/or lossof capacity or even render unintelligible the signals received by thesatellite. In such cases, the solution in the past has been to use twoantennas on the satellite, one for transmission and one for reception ofthe signals. In the case of a very large spacecraft antenna, whichpermits very high capacity and very low user power, it may not bepractical to provide a two-antenna satellite.

FIG. 13 shows an embodiment of the invention which completely eliminatesthe PIM interference problem. One satellite 300 is used to transmit tothe mobile user and a second satellite 301 is used to receive thesignals from the mobile user. Such satellites could be paired togetherat one orbital location, or they could be separated, but both must bewithin the field of view of the user. If specific system designconsiderations indicate that a third satellite should also be used, thentwo satellites could be used to transmit, and one could be used toreceive (or vice versa), again resulting in no problems with PIMproducts. This aspect of the invention can be extended to any practicalnumber of satellites.

Yet another aspect of the invention, which reduces but does notcompletely eliminate PIM effects with a frequency duplexed system, is amultiple satellite system in which non overlapping portions of thesignal bands are allocated to the different satellites, as shown in FIG.14. The lowest order of intermodulation (IM) product is given by:

    Lowest order=integer(|Ftx-Frx|)*2/Band Width+1

If more than one (n>1) satellite is to be launched, a unique portion(most likely 1/n) of the satellite sub-bands OS and IS is assigned toeach of the n satellites. This increases the lowest order of theintermodulation product by a factor of n, and hence greatly reduces themagnitude of the intermodulation power fed into the satellite receiver.

For example, one frequency band under consideration for mobile satelliteservices is 1970-1990 MHZ (satellite receive, Frx) and 2160-2180 MHZ(satellite transmit, Ftx). In this case the lowest order IM product isequal to 20, which would be considered too low for the design of thissatellite service. In the case where a three satellite system is used,if, instead of assigning the entire band to all three satellites, onethird of the band is assigned to each of the three satellites, thelowest IM product is 58, which would be acceptable.

The duplexing technique discussed throughout this application is,obviously, frequency duplexing wherein signals from the user are in adifferent band from the signals to the user. This is the situation shownin FIG. 2. An alternate duplexing method which will completely eliminateany PIM problem is to use time duplexing of the signals to and from thesatellite instead of frequency duplexing the signals. PIM is eliminatedsince the satellite transmits and receives at different times. When timeduplexing at the satellite is used to resolve a potential satellite IMproblem, the user's unit can be either time duplexed or frequencyduplexed. In both cases, the satellite transmit and receive signals mustnot overlap in time.

In this invention, since the user's position and the satellite positionare known the distance from the user to the satellite is computed andhence the transit time for signals can be computed. The user unit'stransmissions are then timed to arrive at the satellite withindesignated receive time slots, with no loss of communications capacity.The satellite power is doubled, but it transmits only half the time.FIG. 10 shows that the signal is buffered 306 into a frame which has afixed number of bits for transmission and requires a time T_(i/o) 302.The signal is encoded with a time frame of t_(e). Buffering in thetransmit buffer requires a time T_(tx). FIG. 10 depicts a system with azero transmission time delay. There is, then, a final time delay T_(d)before the signal fills the out buffer 307. As represented in FIG. 11,with a geosynchronous satellite, there is a round trip transit time offrom 238.6 to 277.8 ms. In this aspect of this invention the satelliteis time duplexed and the user unit is frequency duplexed. It can be seenthat the satellite can transmit and receive on a 50% duty cycle.

In yet another aspect of the invention, both the satellite and the userunits can be timed duplexed FIG. 12. In this case, guard bands must beprovided to make certain that transmit and receive signals do notoverlap at either the satellite or the user unit. The example shownrelates to a geosynchronous satellite. In this aspect of this invention,the satellite can transmit on a 41% duty cycle.

In one embodiment, and as shown in FIGS. 1-2, a mobile user's unit 22will send an occasional burst of an identification signal in the ICsub-band either in response to a poll or autonomously. This may occurwhen the unit 22 is in standby mode. This identification signal istracked by the regional node control center 14 as long as the unit iswithin that respective region, otherwise the signal will be tracked bythe satellite node or nodes. In another embodiment, this identificationsignal is tracked by all ground and satellite nodes capable of receivingit. This information is forwarded to the network control center 12 viastatus and command lines. By this means, the applicable regional nodecontrol center 14 and the system network control center 12 remainconstantly aware of the cellular location and link options for eachactive user 22. An intra-regional call to or from a mobile user 22 willgenerally be handled solely by the respective regional node controlcenter 14. Inter-regional calls are assigned to satellite or groundregional system resources by the system network control center 12 basedon the location of the parties to the call, signal quality on thevarious link options, resource availability and best utilization ofresources.

A user 22 in standby mode constantly monitors the common outboundcalling frequency sub-band OC 32 for calling signals addressed to him bymeans of his unique spreading code. Such calls may be originated fromeither ground or satellite nodes. Recognition of his unique call codeinitiates the user unit 22 ring function. When the user goes "off-hook",e.g., by lifting the handset from its cradle, a return signal isbroadcast from the user unit 22 to any receiving node in the usercalling frequency sub-band IC 38. This initiates a handshaking sequencebetween the calling node and the user unit which instructs the user unitwhether to transition to either satellite, or ground frequencysub-bands, OS 30 and IS 36 or OG 28 and IG 34.

A mobile user wishing to place a call simply takes his unit 22 off hookand dials the number of the desired party, confirms the number and"sends" the call. Thereby an incoming call sequence is initiated in theIC sub-band 38. This call is generally heard by several ground andsatellite nodes which forward call and signal quality reports to theappropriate system network control center 12 which in turn designatesthe call handling to a particular satellite node 20 or regional nodecontrol center 14. The call handling element then initiates ahandshaking function with the calling unit over the OC 32 and IC 38sub-bands, leading finally to transition to the appropriate satellite orground sub-bands for communication.

Referring now to FIG. 3, a block diagram of a communications system 40which does not include a system network control center is presented. Inthis system, the satellite node control centers 42 are connecteddirectly into the land line network as are also the regional nodecontrol centers 44. Gateway systems 46 are also available as in thesystem of FIG. 1, and connect the satellite communications to theappropriate land line or other communications systems. The user unit 22designates satellite node 48 communication or ground node 50communication by sending a predetermined code. Alternatively, the userunit could first search for one type of link (either ground orsatellite) and, if that link is present, use it. If that link is notpresent, use the alternate type of link.

Referring now to FIG. 4, a hierarchial cellular structure is shown. Apair of clusters 52 of ground cells 54 are shown. Additionally, aplurality of satellite cells 56 are shown. Although numerals 54 and 56point only to two cells each, this has been done to retain clarity inthe drawing. Numeral 54 is meant to indicate all ground cells in thefigure and similarly numeral 56 is meant to indicate all satellitecells. The cells are shown as hexagonal in shape, however, this isexemplary only. The ground cells may be from 3 to 15 km across althoughother sizes are possible depending on user density in the cell. Thesatellite cells may be approximately 200-500 km across as an exampledepending on the number of beams used to cover a given area. As shown,some satellite cells may include no ground cells. Such cells may coverundeveloped areas for which ground nodes are not practical. Part of asatellite cluster 58 is also shown. The cell members of such a clustershare a common satellite node control center 60.

A significant advantage of the invention is that by the use of spreadspectrum multiple access, adjacent cells are not required to usedifferent frequency bands. All ground-user links utilize the same twofrequency sub-bands (OG 28, IG 34) and all satellite-user links use thesame two frequency sub-bands (OS 30, IS 36). This obviates an otherwisecomplex and restrictive frequency coordination problem of ensuring thatfrequencies are not reused within cells closer than some minimumdistance to one another (as in the FM approach), and yet provides for ahierarchial set of cell sizes to accommodate areas of significantlydifferent subscriber densities.

Referring again to FIG. 1 as well as to FIG. 4, the satellite nodes 20make use of large, multiple-feed antennas 62 which in one embodimentprovide separate, relatively narrow beamwidth beams and associatedseparate transmitters for each satellite cell 56. For example, themultiple feed antenna 62 may cover an area such as the United Stateswith, typically, about 100 satellite beams/cells and in one embodiment,with about 200 beams/cells. As used herein, "relatively narrowbeamwidth" refers to a beamwidth that results in a cell of 500 km orless across. The combined satellite/ground nodes system provides ahierarchical geographical cellular structure. Thus within a densemetropolitan area, each satellite cell 56 may further contain as many as100 or more ground cells 54, which ground cells would normally carry thebulk of the traffic originated therein. The number of users of theground nodes 16 is anticipated to exceed the number of users of thesatellite nodes 20 where ground cells exist within satellite cells.Because all of these ground node users would otherwise interfere asbackground noise with the intended user-satellite links, in oneembodiment the frequency band allocation may be separated into separatesegments for the ground element and the space element as has beendiscussed in connection with FIG. 2. This combined, hybrid service canbe provided in a manner that is smoothly transparent to the user. Callswill be allocated among all available ground and satellite resources inthe most efficient manner by the system network control center 12.

An important parameter in most considerations of cellular radiocommunications systems is the "cluster", defined as the minimal set ofcells such that mutual interference between cells reusing a givenfrequency sub-band is tolerable provided that such "co-channel cells"are in different clusters. Conversely all cells within a cluster mustuse different frequency sub-bands. The number of cells in such a clusteris called the "cluster size". It will be seen that the "frequency reusefactor", i.e., the number of possible reuses of a frequency sub-bandwithin the system is thus equal to the number of cells in the systemdivided by the cluster size. The total number of channels that can besupported per cell, and therefore overall bandwidth efficiency of thesystem is thus inversely proportional to the cluster size. By means tobe described, the invention system achieves a minimum possible clustersize of one as compared to typically 7 to 13 for other ground orsatellite cellular concepts and thereby a maximum possible frequencyreuse factor. This is a major advantage of the invention.

Referring now to FIG. 5, a block diagram is shown of a typical user unit22 to satellite 20 to satellite node control 18 communication and theprocessing involved in the user unit 22 and the satellite node control18. In placing a call for example, the handset 64 is lifted and thetelephone number entered by the user. After confirming a display of thenumber dialed, the user pushes a "send" button, thus initiating a callrequest signal. This signal is processed through the transmitterprocessing circuitry 66 which includes spreading the signal using acalling spread code. The signal is radiated by the omni-directionalantenna 68 and received by the satellite 20 through its narrow beamwidthantenna 62. The satellite processes the received signal as will bedescribed below and sends the backhaul to the satellite node controlcenter 18 by way of its backhaul antenna 70. On receive, the antenna 68of the user unit 22 receives the signal and the receiver processor 72processes the signal. Processing by the user unit 22 will be describedin more detail below in reference to FIG. 7.

The satellite node control center 18 receives the signal at its antenna71, applies it to a circulator 73, amplifies 74, frequency demultiplexes76 the signal separating off the composite signal which includes thesignal from the user shown in FIG. 5, splits it 78 off to one of a bankof code correlators, each of which comprises a mixer 80 for removing thespreading and identification codes, an AGC amplifier 82, the FECCdemodulator 84, a demultiplexer 86 and finally a voice encoder/decoder(CODEC) 88 for converting digital voice information into an analog voicesignal. The voice signal is then routed to the appropriate land line,such as a commercial telephone system. Transmission by the satellitenode control center 18 is essentially the reverse of the above describedreception operation.

Referring now to FIG. 6, the satellite transponder 90 of FIG. 5 is shownin block diagram form. A circulator/diplexer 92 receives the uplinksignal and applies it to an L-band or S-band amplifier 94 asappropriate. The signals from the M satellite cells within a "cluster"are frequency multiplexed 96 into a single composite K-band backhaulsignal occupying M times the bandwidth of an individual L-/S-band mobilelink channel. The composite signal is then split 98 into N parts,separately amplified 100, and beamed through a second circulator 102 toN separate satellite ground cells. This general configuration supports anumber of particular configurations various of which may be best adaptedto one or another situation depending on system optimization which forexample may include considerations related to regional land line longdistance rate structure, frequency allocation and subscriber population.Thus, for a low density rural area, one may utilize an M-to-1 (M>1, N=1)cluster configuration of M contiguous cells served by a single commonsatellite ground node with M limited by available bandwidth. In order toprovide high-value, long distance service between metropolitan area,already or best covered for local calling by ground cellular technology,an M-to-M configuration would provide an "inter-metropolitan bus" whichwould tie together all occupants of such M satellite cells as if in asingle local calling region. To illustrate, the same cells (for example,Seattle, Los Angeles, Omaha and others) comprising the cluster of M usercells on the left side of FIG. 6, are each served by correspondingbackhaul beams on the right side of FIG. 6.

Referring now to FIG. 7, a functional block diagram of a typical userunit 22 is shown. The user unit 22 comprises a small, light-weight,low-cost, mobile transceiver handset with a small, non-directionalantenna 68. The single antenna 68 provides both transmit and receivefunctions by the use of a circulator/diplexer 104 or other means. It isfully portable and whether stationary or in motion, permits access to awide range of communication services from one telephone with one callnumber. It is anticipated that user units will transmit and receive onfrequencies in the 1-3 Ghz band but can operate in other bands as well.

The user unit 22 shown in FIG. 7 comprises a transmitter section 106 anda receiver station 108. For the transmission of a voice communication, amicrophone couples the voice signal to a voice encode 110 which performsanalog to digital encoding using one of the various modern speech codingtechnologies well known to those skilled in the art. The digital voicesignal is combined with local status data, and/or other data, facsimile,or video data forming a composite bit stream in digital multiplexer 112.The resulting digital bit stream proceeds sequentially through forwarderror encoder 114, symbol or bit interleaver 116, symbol or bit, phase,and/or amplitude modulator 118, narrow band IF amplifier 120, widebandmultiplier or spreader 122, wide band IF amplifier 124, wide band mixer126, and final power amplifier 128. Oscillators or equivalentsynthesizers derive the bit or baud frequency 130, pseudo-random noiseor "chip" frequency 132, and carrier frequency 134. The PRN generator136 comprises deterministic logic generating a pseudo-random digital bitstream capable of being replicated at the remote receiver. The ringgenerator 138 on command generates a short pseudo-random sequencefunctionally equivalent to a "ring".

The transceiver receive function 108 demodulation operations mirror thecorresponding transmit modulation functions in the transmitter section106. The signal is received by the non-directional antenna 68 andconducted to the circulator 104. An amplifier 142 amplifies the receivedsignal for mixing to an IF at mixer 144. The IF signal is amplified 146and multiplied or despread 148 and then IF amplified 150 again. The IFsignal then is conducted to a bit or symbol detector 152 which decidesthe polarity or value of each channel bit or symbol, a bit or symbolde-interleaver 154 and then to a forward error decoder 156, thecomposite bit stream from the FEC decoder 156 is then split into itsseveral voice, data, and command components in the de-multiplexer 158.Finally a voice decoder 160 performs digital to analog converting andresults in a voice signal for communication to the user by a speaker orother means. Local oscillator 162 provides the first mixer 144 LO andthe bit or symbol detector 152 timing. A PRN oscillator 164 and PRNgenerator 166 provide the deterministic logic of the spread signal fordespreading purposes. The baud or bit clock oscillator 168 drives thebit in the bit detector 152, forward error decoder 156 and the voicedecoder 160.

The bit or symbol interleaver 116 and de-interleaver 154 provide a typeof coded time diversity reception which provides an effective power gainagainst multipath fading to be expected for mobile users. Its functionis to spread or diffuse the effect of short burst of channel bit orsymbol errors so that they can more readily be corrected by the errorcorrection code.

As an alternative mode of operation, provision is made for direct dataor facsimile or other digital data input 170 to the transmitter chainand output 172 form the receiver chain.

A command decoder 174 and command logic element 176 are coupled to theforward error decoder 156 for receiving commands or information. Bymeans of special coding techniques known to those skilled in the art,the non-voice signal output at the forward error decoder 156 may beignored by the voice decoder 160 but used by the command decoder 174. Anexample of the special coding techniques are illustrated in FIG. 7 bythe MUX 112 and DEMUX 158.

As shown, acquisition, control and tracking circuitry 178 are providedin the receiver section 108 for the three receive side functionaloscillators 162, 164, 168 to acquire and track the phase of theircounterpart oscillators in the received signal. Means for so doing arewell known to those skilled in the art.

The automatic gain control (AGC) voltage 184 derived from the receivedsignal is used in the conventional way to control the gain of thepreceding amplifiers to an optimum value and in addition as an indicatorof short term variations of path loss suffered by the received signal.By means to be described more in detail below, this information iscombined with simultaneously received digital data 186 in a power levelcontroller to a value such that the received value at the satellite nodecontrol is approximately constant, independent of fading and shadowingeffects. The level commanded to the output power to the output poweramplifier 128 is also provided 190 to the transmitter multiplexer 112for transmission to the corresponding unit.

In mobile and other radio applications, fading, shadowing, andinterference phenomena result in occasional, potentially significantsteep increases of path loss and if severe enough, may result in dataloss. In order to insure that the probability that such a fade will bedisruptive is acceptably low, conventional design practice is to providea substantial excess power margin by transmitting at a power level thatis normally as much as 10 to 40 dB above the average requirement. Butthis causes correspondingly increased battery usage, inter-system, andintra-system interference. In a CDMA application, this can drasticallyreduce the useful circuit capacity of the channel.

A further feature of a system in accordance with the principles of theinvention is an adaptive two-way power control system which continuallymaintains each transmitted signal power at a minimum necessary level,adapting rapidly to and accommodating such fades dynamically, and onlyas necessary. In controlling the transmitted signal power, the adaptivepower control system in accordance with the invention comprises two mainadaptive sections, the first being an adaptive path loss power controlsystem and the second being an adaptive signal quality power controlsystem. The adaptive power control system in accordance with theinvention considers not only path loss, but also a measure of data lossof "signal quality" reported to it from another unit with which it is incommunication. As used herein, "signal quality" refers to the accuracyor fidelity of a received signal in representing the quantity orwaveform it is supposed to represent. In a digital data system, this maybe measured or expressed in terms of bit error rate, or, if variable,the likelihood of exceeding a specified maximum bit error ratethreshold. Signal quality involves more than just signal strength,depending also on noise and interference level, and on the variabilityof signal loss over time. Additionally, "grade of service" as usedherein is a collective term including the concepts of fidelity,accuracy, fraction of time that communications are satisfactory, etc.,any of which may be used to describe the quality objectives orspecifications for a communication service. Examples of grade of serviceobjective would include:

bit error rate less than one in 10³ ;

ninety percent or better score on the voice diagnostic rhyme test; and

less than one-half percent probability of fade below threshold,

although the exact numbers may vary depending on the application.

Power adjustment based upon path loss reciprocity alone is subject toseveral sources of error, including, path non-reciprocity (due tofrequency difference), staleness due to transmit time delay, and localnoise or interference anomalies. Compensation for all these effects isprovided in the system and method of the invention by a longer termsignal quality monitor, which compares recent past actual error ratestatistics, (measured in the forward error correction decoder) andcompares against prescribed maximum acceptable error rate statistic. Thedifference is interpreted as a longer-term signal level deficiency,where it is used to provide a longer term supervisory control over theshort term path-reciprocity power adjustment system. Thus, for example,if a mobile terminal passes into an urban area where it suffersdeep-fast fades that cannot be fully compensated due to the delay in thepath reciprocity sensing power control, the longer term signal qualitydeficiency estimate will sense this and call for a gradual increase inthe reference value calibration of the fast, signal sensing powercontrol.

Discussing now an embodiment of the adaptive path loss power controlsystem, each transmitter telemeters its current signal output level tothe counterpart far end receiver by adding a low rate data stream to thecomposite digital output signal. Using this information along with themeasured strength of the received signal and assuming path lossreciprocity, each end can form an estimate of the instantaneous pathloss and adjust its current transmit power output to a level which willproduce an approximately constant received signal level at thecounterpart receiver irrespective of path loss variations.

Referring now to FIGS. 8(a) through 8(h), timing and waveform diagramsof the adaptive path loss system of an adaptive power control system inaccordance with the principles of the invention are presented. In thisexample, the two ends of the communications link are referred togenerally as A and B. In the ground cellular application, "A"corresponds to the user and "B" corresponds to the cellular node. In thesatellite link, A would be the user and B would be the satellite controlnode; in this case, the satellite is simply a constant gain repeater andthe control of its power output is exercised by the level of the signalsent up to it.

In the example of FIG. 8(a), at time 192, the path loss suddenlyincreases x dB due for example to the mobile user A driving behind abuilding or other obstruction in the immediate vicinity of A. Thiscauses the signal strength as sensed by A's AGC to decrease x dB asshown in FIG. 8(b). The telemetered data at time 92 shown in FIG. 8(c)indicates that the level at which this signal had been transmitted fromB had not been altered, A's power level controller 188 subtracts thetelemetered transmitted signal level from the observed received signallevel and computes that there has been an increase of x dB in path loss.Accordingly it increases its signal level output by x cB at time 192 asshown in FIG. 8(d) and at the same time adds this information to itsstatus telemeter channel.

This signal is transmitted to B, arriving after transit time T as shownin FIG. 8(e). The B receiver sees a constant received signal strength asshown in FIG. 8(f) but learns from the telemetered data channel as shownin FIG. 8(g) that the signal has been sent to him at +x dB. Therefore, Balso computes that the path loss has increased x dB, adjusts its outputsignal level accordingly at FIG. 8 (h) and telemeters that information.That signal increases arrives back at stations A at 2T as shown in FIG.8(e) thus restoring the nominal signal strength with a delay of twotransit times (T). Thus for a path loss variation occurring in thevicinity of A, the path loss compensation at B is seen to be essentiallyinstantaneous while that at A occurs only after a two transit timedelay, 2T.

FIG. 9 shows the operation of an adaptive signal quality power controlsystem acting in concert with the adaptive path loss power controlsystem described above. While FIG. 9 depicts only one of twocorresponding transceivers 210 which are in communication with eachother, the one not shown functions identically to the one shown in FIG.9 and described. Receiver 212 receives the signal from the correspondingtransceiver and provides a measure indicative of the near-end receivedsignal level deviation from a nominal level 214 by techniques well knownto those skilled in the art as a step in determining the path loss. Thenominal level is typically calculated to provide a desired minimumacceptable grade of service under average conditions of fading andinterference, as is well known to those skilled in the art. The receiver212 provides a digital output signal 213 based on the received signal.Forward error decoder 216 decodes the digital information in thereceived signal 213, and in the process provides an error rate measure218, derived from the fraction of transmitted bits needing correction.The forward error decoded signal 218 is further processed in the signalquality circuit 220 to derive signal quality deficiency; i.e., anestimate of the change in transmit power calculated as that which wouldbe required to just achieve the specified, minimum acceptable error rateunder average conditions of fading and interference. The output from thesignal quality circuit 220 is provided to an analog-to-digital converter221 to provide a digital signal to be multiplexed 244. If the error rateis higher than acceptable, the signal quality circuit output 222 willinclude a power increase command signal and if the error rate is lessthan acceptable, a transmit power reduction will be output.

The circuit of FIG. 9 also includes a consideration of thesignal-to-noise ratio (SNR) in the received signal to determine signalquality. The SNR of the received signal is determined in the receiver212 by techniques well known to those skilled in the art; for example,the AGC is monitored, and an SNR signal 223 is provided to the signalquality circuit 220, In this embodiment, the signal quality circuit 220considers both the error rate 218 and the SNR when producing its outputcontrol signal 222.

A demultiplexer 224 separates the telemetered data 217 output throughthe forward error decoder 216 as to far-end signal quality deficiency226, far-end transmitter power deviation reference 228 from a nominallevel, and the traffic signals 230. The far-end transmit power deviationsignal 228 is combined 232 with the near-end received signal leveldeviation 214 to yield a signal 234 representative of the path lossdeviation from a nominal reference value. The telemetered far-end signalquality deficiency 226 and the path loss deviation 234 are combined 236through complementary filters 238 and 240 to yield the transmit powercontrol signal 242 for controlling the output power of the associatedtransmitter 250. The transmit power control signal 242 is also appliedto an analog-to-digital converter 243 to provide a digitized transmitpower control signal 245. The resulting transmitter power leveldeviation from nominal reference 245 and the near-end signal quality 222deficiency signals are multiplexed 244 with the traffic 246, thenforward error encoded 248 and transmitted 250 to the far end transceiverin support of identical functions performed there. The complementarycombining filters 238 and 240 can be designed as optimal estimatingfilters based upon knowledge of the power requirement signal andmeasurement error statistics using methods well known to those familiarwith estimation theory.

Referring again to FIG. 7, an arrangement is provided for generatingcall requests and detecting ring signals. The ring generator 138generates a ring signal based on the user's code for calling out withthe user unit 22. For receiving a call, the ring signal is detected in afixed matched filter 198 matched to a short pulse sequence which carriesthe user's unique code. By this means each user can be selectivelycalled. As an option, the ring detect and call request signals may beutilized in poll/response mode to provide tracking information on eachactive or standby mode user. Course tracking information, adequate formanagement of the call routing functions is provided by comparison ofsignal quality as received at various modes.

For the precision location option, the user response signal time isaccurately locked to the time of receipt of the polling or timingsignal, to a fraction of a PRN chip width. measurement of the round trippoll/response time from two or more nodes or time differences of arrivalat several nodes provides the basic measurement that enable solution andprovision of precise user position. Ground and satellite transmittersand receivers duplicate the functions summarized above for the userunits. Given a priori information, for example as to the route plan of avehicle, a single round trip poll/response time measurement from asingle node can yield valuable user position information.

The command logic 176 is further coupled to the receiver AGC 180, thematched filter ring detector (RD) 198, the acquisition and trackingcircuitry 178, the transmit local oscillator (LO) 162 and the ringgenerator (RG) 138 to command various modes of operation.

The economic feasibility of a mobile telephone system is related to thenumber of users that can be supported. Two significant limits on thenumber of users supported are bandwidth utilization efficiency and powerefficiency. In regard to bandwidth utilization efficiency, in either theground based cellular or mobile satellite elements, radio frequencyspectrum allocation is a severely limited commodity. Measuresincorporated in the invention to maximize bandwidth utilizationefficiency include the use of code division multiple access (CDMA)technology which provides an important spectral utilization efficiencygain and higher spatial frequency reuse factor made possible by the userof smaller satellite antenna beams. In regard to power efficiency, whichis a major factor for the satellite-mobile links, the satellitetransmitter source power per user is minimized by the use offorward-error-correcting coding, which in turn is enabled by the aboveuse of spread spectrum code division multiple access (SS/CDMA)technology and by the use of relatively high antenna gain on thesatellite. CDMA and forward-error-correction coding are known to thoseskilled in the art and no further details are given here.

The issue of bandwidth utilization efficiency will now be considered indetail. The major contribution of SS/CDMA to spectral efficiency isclosely related to the concept of cellular "cluster". In existingFrequency Division or Time division multiple access technology, a givenfrequency or time slot allocation must be protected from interferencefrom nearby cells by users on the same frequency sub-band. Depending onthe degree of protection required, it may be necessary to preclude thereuse of the cell "X" frequencies on a number of cells, N, surrounding"X". That number is called the "cluster size." Because each cell canthen utilize only one Nth of the total allocatable channels, it will beseen, all other things being equal, that the "frequency reuse factor"and spectral utilization efficiency are inversely proportional to thecluster size, N.

Field tests of the FM-frequency division multiplex ground cellularsystem, Macdonald, V. H., The Cellular Concept, Bell Systems TechnicalJournal, p. 15, January 1979, determined that a signal-to-interferenceratio of 17 dB or better is required for good to excellent quality to beperceived by most listeners. This, combined with propagation and fadingstudies, yielded the criterion that the separation between co-channelsites should be at least 6.0 times the maximum distance to a user withinthe cell using omni-directional antennas at the ground nodes. In orderto achieve this separation, the cluster size must be at least N=12 cellsper cluster. Thus one may use only 1/12 of the total allocatablecapacity per cell.

In satellite service, the minimum cell size is inversely proportional tothe satellite antenna dish diameter. For a given maximum feasiblesatellite antenna dish diameter, the number of available channels isstrictly limited by the cluster size. In the planned AMSC system, C. E.Agnew, et al., The AMSC Mobile Satellite System, Proceedings of theMobile Satellite Conference, NASA, JPL, May 1988, the effective clustersize is 5, and one may use only 1/5 of the total allocatable capacityper cell.

In a system in accordance with the invention, the cluster size is one.That is, each cell uses the same, full allocated frequency band. This ispossible because of the strong interference rejection properties ofspread spectrum code division multiple access technology (SS/CDMA). Theeffect of users in adjacent cells using the same band is qualitativelyno different than that of other users in the same cell, so may be takeninto account as an effective reduction in the number of users that canbe tolerated within a cell. The cumulative effect of all such other-cellinterferers may be calculated on the assumption of uniform density ofusers and a distance attenuation law appropriate to the case of groundpropagation or satellite beam pattern. Doing so, we find the multiplyingfactor for the ratio of total interference to in-cell origininterference of 1.4 for ground propagation and 2.0 for the satellitesystem. This factor may be accounted for as a multiplier equivalent ineffect to an effective cluster size for the CDMA system. Thus, finally,it is believed that in comparison with other systems we find frequencyreuse factor or bandwidth utilization efficiency factors inverselyproportional to effective cluster size in the ratios:

    0.71:0.5:0.2:0.08

for respectively the ground cellular component of the invention,satellite cellular component of the invention, the AMSC mobile satelliteconcept, and current ground cellular technology.

The second severely limited commodity in the satellite links issatellite prime power, a major component of the weight of acommunication satellite and thereby a major factor in satellite cost.Generally in systems such as this, the down links to individual usersare the largest power consumers and thus for a limited satellite sourcepower, may provide the limiting factor on the number of users that canbe served. Thus it is important to design the system for minimumrequired power per user. This requirement is addressed in the inventionin four ways. In the invention the system envisages the user of thehighest feasible satellite antenna gain. In one embodiment, power gainon the order of 45 dB and beamwidth of under one-degree are envisionedat L-band. This is accomplished by an antenna size of approximately 20meters.

Secondly, by virtue of the use of the spread spectrum technique, verylow rate high gain coding is available without penalty in terms ofincreased bandwidth occupancy.

Thirdly, the system utilizes channel bit interleaving/de-interleaving, akind of coded time diversity to provide power gain against deep fadingnulls. This makes it possible to operate at relatively low bit energy tonoise density ratio, on the order of 3 dB. This in turn results inminimum satellite power requirements per user.

Fourthly, two-way, adaptive power control and signal quality control aspreviously described obviate the usual practice of continuouslytransmitting at a power level which is 10 to 40 dB greater than requiredmost of the time in order to provide a margin for accommodatinginfrequent deep fades.

In addition to the above listed advantages, the Code Division Multiplexsystem has the following important advantages in the present system.Blank time when some of the channels are not in use reduces the averageinterference background. In other words, the system overloads andunderloads gracefully. The system inherently provides flexibility ofbase band rates; as opposed to FDM system, signals having differentbaseband rates can be multiplexed together on an ad-hoc basis withoutcomplex preplanned and restrictive sub-band allocation plans. Not allusers need the same baseband rate. Satellite antenna sidelobe controlproblems are significantly reduced. The above mentioned numericalstudies of out-of-cell interference factors show that secondary loberesponses may effectively be ignored. Co-code reassignment (that isreuse of the same spreading code) is feasible with just one beamseparation. However, because there are effectively (i.e., includingphasing as a means of providing independent codes) an unlimited numberof channel codes, the requirements on space division are eased; there isno need to reuse the same channel access i.e., spreading code.

By virtue of the above discussed design factors the system in accordancewith the invention provides a flexible capability of providing thefollowing additional special services: high quality, high rate voice anddata service; facsimile (the standard group 3 as well as the high speedgroup 4); two way messaging, i.e, data interchange between mobileterminals at variable rates; automatic position determination andreporting to within several hundred feet; paging rural residentialtelephone; and private wireless exchange.

It is anticipated that the satellite will utilize geostationary orbitsbut is not restricted to such. The invention permits operating in otherorbits as well. The system network control center 12 is designed tonormally make the choice of which satellite or ground node a user willcommunicate with. In another embodiment, as an option, the user canrequest his choice between satellite link or direct ground based linkdepending on which provides clearer communications at the time orrequest his choice based on other communication requirements.

While a satellite node has been described above, it is not intended thatthis be the only means of providing above-ground service. In the casewhere a satellite has failed or is unable to provide the desired levelof service for other reasons, for example, the satellite has been jammedby a hostile entity, an aircraft or other super-surface vehicle may becommissioned to provide the satellite functions described above. The"surface" nodes described above may be located on the ground or in waterbodies on the surface of the earth. Additionally, while users have beenshown and described as being located in automobiles, other users mayexist. For example a satellite may be a user of the system forcommunicating signals, just as a ship at sea may or a user on foot.

Accurate position determination can be obtained through two-dimensionalmulti-lateration. Each CDMA mobile user unit's transmitted spreadingcode is synchronized to the epoch of reception of the pilot signal fromits current control site, whether ground or satellite node. The normalmode of operation will be two-dimensional, i.e., based upon tworeceptions, at ground or satellite nodes of the user response code. Inconjunction with a priori information inherent in a topographicdatabase, e.g., altitude of the surface of the earth, position accuracyto within a fraction of a kilometer can be provided. Other means areavailable for position location, such as GPS, see FIG. 15.

Means for determining the position of a mobile user relative to amultiplicity of known system nodes, either fixed on the ground or atknown positions in space, are known to those skilled in the art. In aCDMA of system, any of these means are largely incidental to thefunction of transmitting and/or receiving the CDMA signal at multiplesites. The receiving function requires synchronization of the epoch of alocal spread code generator to that of the received spread code, so thathaving achieved code synchronization, one inherently has a measure ofthe delay time and hence the range of the signal. Various referencesdescribe how this information can be used in several differentgeometrical configurations to provide the delay measurements necessaryto provide hyperbolic, elliptical, spherical or hybrid multi-laterationposition determination. By any of these means the mobile position caneither be determined by the network controller or by the mobile user andrelayed to the network controller.

Signal theft or "pirating" is a major problem in current cellular and TVreceive only (TVRO) systems, and will probably affect additional futurecommunications systems. Pirates manage to learn a code intended torestrict the system use to the authorized customers for whom it wasintended, and then to alter users units such that they become pirateunits which operate using the stolen code. Thus, unlawful use of thesystem is accomplished and the pirate user does not pay the fees due tothe service provider. In the past, such pirates have been amazinglysuccessful at their unlawful trade, both in terms of speedy delivery totheir markets and the value of stolen signals. Such piracy continues ona large scale today.

This invention utilizes the knowledge of a users position, obtained asdescribed above, first to verify the legitimate, authorized users unit.The remaining units operating on that code are clearly identified as nonpaying or pirate users by virtue of having a different position. In oneembodiment, this information is used to apprehend the pirates.

In an alternate embodiment, the pirated unit can be disabled. There aretwo embodiments for disabling the pirated unit, each being effectiveunder different circumstances. The first involves simply not providingservice. The second involves commanding the disablement of the piratedunit by means including commanding that fuses to be blown within thecircuitry and commanding the destruction of the user circuitry.

FIG. 16 depicts a hierarchial control division along geographical andground vs. satellite elements of a mobile system. In this diagram solidlines denote traffic flow, dotted lines command, status and controlflow.

The total number of national, much less worldwide circuits is so vastthat maximum decentralization of control is desired and accomplished bythis control hierarchy invention. Every command and allocation decisionshould be made at the lowest level at which all the necessaryinformation to make the decision is available. Thus it is anticipatedthat the bulk of the service requests and handovers coming upward into aparticular level will be resolved at that level, with only thoseinvolving higher or lateral coordination being passed on up the line. Anumerical example of a system using this control hierarchy follows.

A typical ground cell 310 is assumed to comprise two 1.25 MHZ subbands,each serving up to 54 circuits for a total of 108. These are the commonground cells. Geographically they may be thought of as from 3 to 12miles in diameter. About 6 of the 108 circuits are reserved for callingchannels. Power control functions with ground cellular users are theonly functions handled here. No other switching or control functions areperformed here, but all traffic lines are trunked on up the line alongwith station status.

Depending on local demographics, up to about 50 (more typically 20) suchground cell trunks comprise a GND METRO control 311, or Mobile TelephoneSwitching Office (MTSO). This would correspond roughly with ametropolitan area such as greater Los Angeles. Local calls and handoverswithin the ground metro area would be resolved and controlled at thislevel. As a mobile user travels towards the edge of the metro area thiswill be recognized by the fact that he is being served by one of theouter rank of "edge" cells. For any user in these cells any signal droprequiring handover will be coordinated at regional level with theappropriate adjacent metro or satellite cell. Generally the GND METROregions will be made coterminous with the satellite cells provided thereare any ground cells within the satellite cell.

The SATELLITE CELLS corresponding to satellite beams, might be about 200mi (normal to beam) in diameter, and provide about 741 circuits of whichsome 200 are calling channels. Control functions correspond to those ofthe Ground cell.

The REGIONAL control 312 areas in one embodiment are coterminous withthe satellite "Clusters", typically about 10 satellite cells or about600 miles diameter. Each may handle 1 to 100 (typically 15) METROregions.

The SATELLITE CLUSTER CONTROL as part of the regional control handlesabout 10 satellite cells. The facility is collocated with the GroundRegional Control facility.

The NATIONAL NET CONTROL 314 handles about 15 Regional centers for thecase of a United States National system. This comprises all of thefacilities envisioned in the present application.

Control functional allocation among these various control levels can beas follows:

Ground Cell 310 and Satellite Cell 315: Power control, Time-of-Arrivalmeasurement and reporting as assigned (basis for positiondetermination), detect monitor and report up all current standbys andcall requests, and call terminations in coverage area, handle traffic asassigned including handshaking and call establishment, and disconnect.Each cell has a level at which saturation occurs, i.e., a limit on howmany bits of information can be communicated through that node. Theinstantaneous information being transmitted through any node can bemeasured by the instantaneous output power level at each of thetransmitters associated with each node, and/or the instantaneousreceived power level at each of the receivers associated with each node.Alternatively, the number of calls being instantaneously handled isknown at the control centers. A measure of this information can be sentto any or all users such that they could delay transmission until a timewhen the use is low and hence receive more favorable rates. Theinformation can be displayed by lamps or LCD or other means to permitmanual decision making, e.g., whether or not to place a call. In analternative embodiment, the information could be automatically used toenable transmission, e.g., for data or fax transmission.

Ground Metro Control 311: Coordinates soft handovers between groundcells, TELCO interface for ground links.

Regional Control 312: Provides TELCO interface to satellite links,tracks position of all active or standby units in region; assignstraffic handling facility and subband (coordinating exclusion areas),and forwards up requests for handovers out of the region and requestsfor additional resources for position fixing.

National Control 314: in one embodiment provides; Satellite statusmonitoring; orbit maintenance, power; management, spares control;satellite housekeeping; coordinates position fixing resources asrequested; and coordinates interregional handovers.

One embodiment of a method and protocol for the handover of a mobileuser from one operator to another is described below (with reference toFIG. 17):

3. Whenever the recomputed algorithm should call for a transfer from"old" to any other, "new" service provider, the following events wouldoccur:

a) Old provider sends a formatted message 316 to new provider, meaning:

"Request transfer to your system of User N at location XYZ, nowconnected to user K, code assignments UVW, whose traffic is hereby beingbridged to you via landline circuit PDQ of trunk ABC" . . . and anyother information that may be useful in call setup on new system.

b) New system sends message 317 to old meaning:

"call can be accepted, assign calling channel L, code XY, subband S . .."

c) Old system sends a command 318 to subscriber set:

"Initiate transfer to new system on new system calling channel L, codeXY, subband S . . ." and any other information which would expedite thetransfer.

d) New system assigns a termination unit to acquire user N on hiscalling channel L, code XY, subband S . . . and makes the landlineconnection to circuit PDQ of trunk ABC . . . 319.

e) Subscriber and new system do as much as possible of call initiationhandshaking i.e., "initial setup", while maintaining traffic via oldsystem. In one embodiment the other (non-handling-over) party isconnected and the forward direction signals are being transmitted fromboth old and new systems simultaneously in parallel; the necessary usercommand signals (like new frequency, new spreading code, . . .) havebeen sent to him and are stored in registers ready to be but not yetexecuted; and the new reverse direction receiver terminal unit isassigned, standing by, searching for the anticipated signal code, andits output bridged into the reverse direction landline to the otherparty.

f) When initial setup is all ready and standing by, new system messages320 old system:

"Ready to transfer."

g) Old system signals 321 to subscriber unit

"Execute".

h) Subscriber unit while continuing reverse direction transmission toold system, instantly drops receive tracking of old forward signal, andcommences receive search to new pilot signal in assigned new frequencyband and code 322. When lock is achieved, normally several 20 ms frameslater, transfers reverse transmission to new band and codes. In oneembodiment the transfer command to the user is synchronized to breaks inthe forward direction utterances to minimize user perceived disruption,and similarly, after the subscriber unit achieves lock and sync on thenew pilot, his transmission transfer is delayed to the next break in hisutterances. In this manner it may be that the several tens ofmilliseconds required for resync can be made essentially transparent toboth user and other party.

i) When new system achieves lock-on to user's reverse transmission 323,new system signals old system to drop the connection.

While several particular forms of the invention have been illustratedand described, it will be apparent that various modifications can bemade without departing from the spirit and scope of the invention.Accordingly, it is not intended that the invention be limited, except bythe appended claims.

Having described the invention in such terms as to enable those skilledin the art to make and use it, and having identified the presentlypreferred best modes thereof, I claim:
 1. A cellular communicationsystem comprising:at least on space node having a multiple beam antennapositioned so as to establish a first set of cells, each antennaincluding means for transmitting and receiving radio signals; at leastone surface node positioned so as to establish a second set of cells,each surface node including means for transmitting and receiving radiosignals; a plurality of user units positioned within the first or secondset of cells, each user unit including means for transmitting andreceiving radio signals for communicating with at least one space nodeand at least one surface node and response means for establishingselective communication with at least one of said nodes; and said userunits including link control means for designating communicationsbetween said user unit and said space nodes or said surface nodes,wherein said link control means permits the user of the user unit todesignate communications between said user unit and said space node orsaid surface node.